Camera shutter



May *2, 1944- -s. ARoNol-F 2,347,700

CAMERA SHUTTER Filed Feb. 1e, 1942 PowEQ F76- SUPPLY -ao 5l 3 b 3l 27 v550 52 35 2a 49 2 a7 a4 I SAMUEL 4pc/vaan:

Patented May 2, 1944 UNITED STATES 'PATENT OFFICE cAMEaasnUr'rEn Samuel Aronoi'f, Dayton, Ohio Application February 1'6, 1942, Serial No.l 431,119

11 claims. (ci. sis-s3) (Granted under the act .of March 3, 1883, as I intended April 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for Government for governmental purposes, without the payment to me of any .royalty thereon.

This invention relates to improvements in camera shutters', and more .particularly to those shutters which are suitable for use on large, highspeed cameras of the type used in aerial photography.

The present-day vaerial cameras require large shutters capable of operating at high speeds and with great Iaccuracy. It has been found very difficult to obtain such a shutter, this being mainly due to the fact that the moving parts are quite large and hence possess considerable inertia. The use of powerful springs for the purpose of overcoming this resistance to change inmovement is not a satisfactory solution to the problem, due to the increased size of the parts required to withstand the strain imposed upon them by the heavier springs. Diiiiculty is also experienced in securing accurate exposure times with the conventional shutters because of variations in the strength of the springs which cause corresponding variations in the operating speed of the shutters. It is particularly important in the case of high-speed shutters that the shutter speeds 'be held within ve'ry'close limits, since the-shorter the exposure, the greater will be the error occasioned by a small variation in the time of exposure. For example, in a shutter operating at one one-thousandth of a' second, a variation of one one-thousandth of a second in its speed of operation will result in a halving or a doubling of the exposure.- This, of course, will result in an underexposed or an overexposed negative andl may mean the loss of an extremely valuable negative which cannot be replaced.

with e. view to overcoming these dimeuities, I

have devised a new type of shutter which does not depend upon springs fory its source of motivation and in which the problem of inertia is no longer present. This is made possible through the use of a constantly rotating disk having an aperture which passes through., the optical axis and uncovers the optical system for a. brief instant on .each revolution oi the disk. In this manner it is possible to secure very-accurately timed exposures so long as the disk rotates with a constant speed. A supplementary screening disk is employed for the purpose of keeping the Sys- `tern covered at all times except when an exposure is to be made. At such time, the screening disk is movedso as to uncover the system during the instant that it is uncovered by the rotating disk, after which the screening disk again covers the system so as to prevent a double exposure as the aperture in the disk comes around again. Thus, a very rapid and accurate exposure may be made in a camera of unlimited size by the use of my mechanism.

Accordingly, one of the objects of my invention is to provide a high-speed shutter suitable for use on large cameras of the type used in aerial photography. In so doing, I have devised a means for making very rapid exposures with extreme accuracy, the means employed being quite simple in nature and very reliable in operation.

Another object of my invention is to provide a high-speed shutter vfor cameras in which the exposure is made by the uncovering of the optical. v system by an aperture located near the periphery of a rapidly rotating disk.

A further object of my invention is to provide a high-speed shutter for cameras in which a screening disk is employed in conjunction with the rotating disk for the purpose of covering the optical system at all times except for the brief instant when an exposure is being made.

Still a further object of my invention is to provide a high-speed shutter for cameras in which means is provided for synchronizing the movement of the screening disk with that of the rotating disk so that when an exposure is made. both will uncover the optical system at the same instant. f

In general, my device is comprised of a rapidly rotating disk which has provided therein an aperture so located as to pass through the optical axis oi' the system on each rotation of the disk. In this manner it is possible to obtain very high exposure speeds and at the same time to secure a very high degree of accuracy in exposure times. In order to prevent an exposures being made on each rotation oi the disk as the aperture passes through the optical axis. a screening plate is provided beneath the rotating disk for the purpose of keeping the system covered atall times except when an exposure is to be made. This screening plate contains an aperture similar to the one in the disk but lying normally outl of line therewith except at such times as an exposure is to be made. At this time the plate is moved in such amanner as lto'cause the aperture therein to pass through the optical axis at the same instant that the aperture in the rotating disk passes there.. through. This allows a. momentary beam 0f light to pass through the optical system, thus causing an exposure to be made. The means Vfor moving the plate is synchronized with the movement of the rotating disk so as to cause the aperture in the plate to pass through the optical. aids at the same instant that the apertru'e in the disk. passes therethrough. In order to prevent a second exposure of the iilm from taking place as the screening plate is restored to its original. position, a resetting plate is provided beneath. the screening plate for the purpose oi covering the aperture therein during the restoring movement.

The shutter hereinafter described. is inodication of the camera shutter illustrated and de scribed in oopending application llerial ll'o. 43i,ll"19, `filed on February i6, MM2, by Magnus and Arci-lofi. For this reason, no claim is made in. the instant application to the broad aspects' ci' c ntion, this` subiect 'matter heine; claimed non ci Magnus and ny invention "with I lng their normal rotational positie spaced Widely apart a vertical c" order to better show the colistructicn l'ig. 2 shows a modification or the worm and sector synchronizing mechanism illustrated in Fig. l.

.As shown in Fig. l., my improved shutter consists ci a constantly rotating disk l which is seated against and rigidly secured to a 'flange t. formed on a shaft 2. rlhis shaft is rotated with constant speed in a counterclockwise direction by means of a small, directcurren.t motor 3. The disk. I is provided with an aperture 4 so located as to pass through the optical axis 5 of the lens system 6, 'I on each revolution. of the disk. In this manner, the aperture in the disk will be caused to pass through the optical axis from left to right, or in the direction of the arrow E, on each revolution of the disk. Situated below the disk I is a screening plate 9 pivotally mounted on a sleeve lll, the upper end of which serves as a bearing support for the bottom of y the shaft 2. This plate is provided with an aperture Il which, as shown in Fig. 1, normally lies out of line with the optical axis 5 when the plate is in its normal position. The plate 9 is urged by a spring I3 in the direction of the arrow I4, thus tending to move the aperture II through the optical axis in a left-to-right direction as viewed in the figure. Such movement is normally prevented by the engagement o -a nose l5` on the plate 9 against the blunt end I6 of a latch I1 pivotally mounted in the framework of the shutter at I8. The armature I9 ofthe solenoid 20 is pivotally connected to the latch so as to move the latter in a counterclockwise direction when the solenoid is energized, thus releasing the plate B for movement under the influence of the spring I3. Lying below the plate 9 is a resetting plate 2l which is likewise pivotally mounted on the sleeveA IU, this plate being held by a spring 22 in contact with a stationary block 23. vThe plate 2| is of solid construction,'there being no apertures therein, and when in the position shown in the gure, this plate is in such position as not to interfere with the light passing through the optical system of the camera. A resetting knob 24 is provided on the plate 2l in order that the same may be swung by the operator in a clockwise direction when resetting the shutter. This plate is also Cil lll

atraveo provided with an upstanding pin 25 which enigages the forward edge of plate 9 when the shutter is being reset. V

A sector 21 is pivotally mounted on a shaft 28, this sector being provided with teeth 29 adapted for engagementwith a single-threaded worm 33 fast on the shaft 2. The sector is weighted so that it will normally tend to rotate about the shaft 28 in a clockwise direction from the position shown in the figure. Such movement is normally prevented, however, by a nose 3l on the sector engaging with a blunt lace 32 provided on a latch 33. This latch is pivotally mounted on a shaftv 34, supported in the framen Work oi? the shutter, and is urged in a clockwise direction 4by means of the torsion spring 35, one end of which engages with the rear edge ci the latch es at while the other end of the spring ed around a pin 31 mounted in the ainework.. The latch vIl?. is provided A.umh plate Bil so that the operator may, seing ...own on this plate, rock. the latch in a counterclockwise direction to thereby re-- lease the sector 21 so as to permit it to drop into engagement with the worm 30. The shaft :t9 on which the sector 21 is mounted is provided with a slip ring 39 which is engaged by a brush MI electrically connected by the wire 4I with one side of the power supply. Fastened to the lefthand end of the sector 21 is a short rod i2, on the end of which is mounted a brush 43. This brush is arranged to engage with a contact block lll mounted on the end of a rod 45 adjustably mounted in a slot formed in the sector 46. The contact block M is electrically connected by a conductor I1 to one side of the solenoid 20, the other side of which is connected by a conductor 48 to the power supply as shown in the figure. A stop 49 is provided to catch the sector 28 after it has passed through the worm 3D. In order to reset the sector to the position shown in the gure, the block 49 may be pushed rearwardly against the pressure of the compression spring 50 and the sector swung clockwise around into normal position by means of the nger grip 5I thereon. The sector 21- is slightly bent at the point where it is pivotally mounted on the shaft 28 so that the right-hand end of the sector will not strike against the latch 33 as it is being swung around into its original position.

The operation of the device is as follows: With the motor 3 operating to constantly rotate the worm 3; and the disk I in a counterclockwise direction, the shutter may be operated to effect an exposure of the film in the camera by manipulating the latch 33 to releasev the sector 21. The sector will then dropv into engagement with the worm 3U and be carried through the worm as the latter rotates. At a predetermined point in the travel of the sector through the worm, the brush 43 will make contact vwith the block 4I to thereby energize the solenoid 20 and release the screening plate 3 for movement in the direction oi the arrow Il under the urgency of the spring I3. This will carry the aperture II in the plate through the optical axis 5 in a direction from left to right or in the same direction as that pursued by the aperture in the disk I as it passes through the optical axis. The contact block 44 is adjusted along the sector I6 to such a point that contact will be made and the plate 9 released at the proper moment to cause the aperture Il in the plate to pass through the optical axis at the same time that the aperture I in the rotating .circuit for the purpose of disk I passes therethrough. For obtaining maximum duration of exposure, both disk and plate 9 should travel at substantially the same' speed when passing through the optical axis. For obtaining minimum duration of exposure, it is necessary that there'be a maximum difference -Vof plate 9 is passing through the optical axis. The aperture in the plate 9 is carried beyond the optical axis as .the plate continues its rotation and, when the 'plate comes to rest against the pin 25, the aperture will lie out of line with this axis so as to again obstruct the passage of light through the optical 'system. Thus, although the disk I is constantly rotating and will uncover the optical system at .frequent intervals, an exposure can be made only when the latch 33 is manipulated, and the action of the screening plate will insure that only a single exposure is made. y

After exposure, the shutter is reset by the operators grasping the knob 24 of the reset plate 2| and swinging the same in a clockwise direction against the pull of the spring 22. The pin 25 mounted in the reset plate will carry the screening plate along with it, and both plates are thus the screening plate is` again latched by I1. Duringthis operation the plate 2| covers the aperture in the screening plate so that there is no possibility of an exposures being made while the plate is being reset to its normal position.

In order that the shutter speed may be varied as desired, a rheostat 52 is placed in the motor controlling the speed of the motor 3 and thereby also the speed of the disk l.

A modification of the worm-and-sector synchronizing mechanism is shown in Fig. 2. As here shown, a cam 54 replaces the Worm 30 on shaft 2, this cam being formed with an abrupt nose 55 for actuating the follower arm 55.v The cam has integrally formed thereon a collar 51 in which is located a setscrew 58 for enabling the annular position of the cam on the shaft to be varied according to requirements. The follower arm 55 is constructed of a thin strip ormetal which is pivotally mounted at its left-hand end in a vertical slot cut in a stationary metal yblock 59. .The right-hand end of the arm is thus free to be moved up or down about the pivot point located in the left-hand end of the arm, but is denied freedom of movement in a horizontal plane. 'Ihe right-hand end of the arm is provided with a contact button 50 which is adapted, upon occasion, to strike against a similar button provided on the righthand end of a stationary metal arm 5|. The latter arm is pinned at its left-hand end to the stationary block 59 and is insulated therefrom by a block of insulating material 52. T he arm 55 is urged to its lowermost position as shown in Fig. 2 by a light spring 53, a limit pin 54 being provided for the purpose of determining the lower limit of movement of the arm. When the armis in this position it lies immediately in front of the cam 54 and its contact button is directly opposite the button carried by arm 5|. f When the arm 55 is lifted by the operator 'by means of finger piece 55 into its raised position, it will be retained in moved together in a clockwise direction until means of a latch pivoted in the shutter frame at 51. The latch is provided with a shoulder 58 which is moved under the lower edge of arm 55 by a spring -55 when the arm is raised. 'I'he modied synchronizing device is connected into the electrical circuit in the same manner as theI worm and sector mechanism, the conductor 4| being. connected to the block 59, and the conductor 41 being connected to the arm 6|.

The operation of the modied form of mechanism is as follows: The device is set or cocked ready for operation by the operators grasping the linger piece 55 and lifting the arm -55 into its raised position where it is retained by the latch`55. .In this position of the arm, the above the upper face of the cam, and the nose 55 passes idly underneath the arm on each revolution of shaft 2. 'I'he shutter is tripped for operation by the operators pushing rearwardly on the upper end of the latch to free the arm 56 to the pull of the spring 53. If the nose of the cam is passing underneath the arm at bhe time the latch is released, the arm will drop down on top of the cam where it will rest until the nose has passed by, at which time it will continue its downward movement until it strikes the pin '64. When the nose of the cam again Icomes around to the position shown in Fig. 2, the arm 56 will be ilexed by the pressure of the nose thereon and the contacts will be momentarily closed. The time of closing of the contacts with relation to the position of the aperture 4 may be varied by releasing the setscrew 58 and rotating the cam around on the shaft 2.

The spacing between the disk and the plates 9 and 2| in the figure is greatly exaggerated for the purpose of more clearly showing their arrangement and construction. In actual practice these plates would be spaced very closely together with just enough room between them to permit the free operation of the shutter.

While, in the particular embodiment of the invention herein described, the disk is rotated in a counterclockwise' direction. or in the same direction as the plate 9, it is to be understood that the shutter will operate just as satisfactorily with the disk rotating in the reverse direction. Infact,when the disk and plate are rotated in opposite directions a higher shutter speed will result due to the combined e'ect of the disk and plate in controlling the time of exposure. A somewhat diierent effect as regards the exposure of the nlm will result `from the two types of operation, however. In

this position against the urgency of spring 5I by 1| the type of operation wherein the disk and plate both rotate in the same direction, the disk, be-` cause of its greater speed, will control the time of exposure. In other words, the plate may, for all practical purposes. be considered as stationary after its aperture reaches the position vwhere it uncovers the optical system, while the aperture 4 in the disk may be considered as rapidly passing through the optical laxis so as to momentarily uncover the optical system. During the passage of the aperture 4 through the optical axis, the side of the nlm first uncovered by the right-'hand edge of the aperture will be the first to be covered by the left-hand edge thereof, and thus al1 parts of lower edge thereof lies will effect a scissors action on the light passing through the optical system. This will mean that the center oi the negative will be the :first to be uncovered by the apertures and the last to be covered by the edges of the disk and the plate. Hence, the center of the film will be exposed for -.a longer period of time than the edges. and per- Y Iectuniformity of the negative will not be secured. In some cases, of course, this disadvantage may be outweighed by the advantage of the higher shutter speeds obtainable with the disk and the plate moving in opposite directions. n

In lieu of plates 9 and 2| illustrated 1n Fig. l, I may use one circular plate in the approximate position of plate 9, having an aperture spaced radially from shaft l in the same position as aperture Il. This plate would be provided with an extension similar to extension l5 of plate 8, which would abut latch I1 in the same manner as illustrated. However, in this arrangement, springs i3, 22, and block 23 would also ybe omitted. The functions performed by these members would be accomplished by a torsion spring having one end associated with the circular plate and the other end secured to an annular ratchet member mounted on sleeve l0. 'I'he ratchet member ris secured against counterclockwise rotation, but is rotatable clockwise to restore the energy of the spring after release of latch I1 and movement of the screening member.

In operation, this form of the invention would function as follows: Upon actuation of the solenoid 20, the latch l1 disengages extension I5, and, plate 9 rotates one revolution, carrying aperture Il through the optical axis and coming to rest by engagement of extension l5 with latch Il which will have returned to its original 'position during this revolution. In this design, no corresponding plate such as shield 2| is necessary, since the aperture Il does not return through the optical axis. After operation, the energy of the torsion spring is restored by a following-up rotation of the ratchet member.

From the above description of my invention it will be seen that I have provided a camera shutter which is capable of operating at very high exposure speeds, even when made of a size suitable for use in the present-day large aerial cameras. At the same time, the exposures will be of a Very uniform nature, the underlying principles of my invention being such as to eliminate the possibility of any substantial error in the time of exposure. 'This is all accomplished in the simplest possible manner, the shutter being composed of only a few commonplace elements which are herein combined in a novel manner capable of producing the highly beneficial results mentioned above.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In a shutter for controlling the passage of light through the optical system of a camera, the combination of a rotating shutter lying in the path of the light passing through the system, said shutter having an aperture therein which passes through the optical axis of the system with each rotation of the shutter, a screening member likewise lying in the path of the light passing through the system and having a normal position and a moved position, said member being provided with an aperture which lies out of line with the optical axis when said member is in either its normal or its moved position but which passes through the optical axis as said 'is member passes from itsnormal to its moved position, means for moving said member from its normal to its moved position, and means operable in accordance with the movement of said shutter for so controlling said moving means as to cause the aperture in said member to pass through the optical axis at the same time that the aperture in said shutter passes therethrough. 2. In a shutter for controlling the passage of light through the optical system of a camera, the combination of a shutter rotating at a Substantially constant speed and lying in the path of the light passing through the system, said shutter having an aperture therein which passes through the optical axis of the system with each rotation of the shutter, a screening member likewise lying in the path of the light passing through the system and having a normal position and a moved position, said member being provided with an aperture which lies out of line with the optical axis when said member is in either its normal or its moved position but which passes through the optical axis as said member passes from its normal to its moved position, means for constantly urging said member toward its moved position, means for retaining said member in its normal position, and means operable in accordance with the movement oi said shutter for disabling said retaining means at the proper moment to cause said urging means to move the aperture in said member through the optical axis at the same time that the aperture in said shutter passes therethrough. i

3. In a' shutter for controlling the passage of light through the optical system of a camera, the combination of a rotating shutter lying in the path of the light passing through the system, said shuttenhaving an aperture `therein which passes through the optical axis of the system on each rotation of the shutter, a screening member likewise lying in the path of the light passing through the system and having a normal position and a moved position, said member being provided with an aperture which lies out of line with the optical axis when said member is in either its normal or its moved position but which passes through the optical axis as said member passes from its normal to its moved position, means for constantly urging said member toward its moved position, means for retaining said member in its normal position, a driving member moving in synchronism with said rotating shutter, a driven element selectively engageable with said driving member, and means actuated by said element in a predetermined position thereof for disabling said retaining means and thereby causing the aperture in said screening member to pass through the optical axis at the same time that the aperture in said shutter passes therey through.

4. In a shutter for controlling the passage 0f` light through the optical system of a carnera,. the combination of a constantly rotating disk situated with its plane at right angles to the optical axis of the system and lying in the path of the light passing therethrough, said disk having an aperture therein which passes through the optical axis on each rotation of the disk, a screening plate situated in overlapping relationship with respect to said disk and having an aperture lying normally out of line with the optical axis, means for moving said screening plate to carry the aperture therein through the optical axis, and means, operating n accordance with the movement of said disk, for so controlling said moving means as to cause the aperture in said plate to pass through the optical axis at the same time that the aperture in said disk passes therethrough.

5. In a shutter for controlling the passage of light through the optical system of' a camera, the combination of a shaft rotating at a submeans constantly urging said plate toward its moved position, means for retaining said plate in its normal position, a worm on ,said shaft, a

toothed sector urged toward engagement with stantially constant speed and lying parallel to the optical axis of the system, a disk on said shaft having a radius substantially greater than the distance between said shaft and the'optical axis, said disk having an aperture therein whichl passes through the optical axis on each rotation of the disk, a vscreening plate situated in overlapping relationship with respect to said disk and having a normal position and a moved position, said plate being provided with an aperture which lies out toward its moved position, means for retaining f said plate in its normal position, a worm on said shaft, a toothed sector selectively engageable with said Worm, and means actuated by said sector in a predetermined position thereof for disabling said retaining means and thereby causing the aperture in said plate to pass through the optical axis at the same time that the aperture in said disk passes therethrough.

6. In a shutter for controlling the passage of light through the optical system of a camera, the combination of a shaft rotating at a substantially constant speed and lying parallel to the optical axis of the system, a disk on said shaft having a radius substantially greater than the distance between said shaft and the optical axis, said disk' having' an aperture therein which passes through the optical axis on each rotation of the disk, a screening plate situated in overlapping relationship with respect to said disk and having a normal position and a moved position, said plate being provided with an aperture which lies out of line with the optical axis when the plate is in either its normal or its moved position but which passes through the optical axis as the plate moves from its normal to its moved position, spring means constantly urging said plate toward its moved position, means for retaining said plate in its normal position, a cam on said shaft, a follower arm selectively engageablewith said cam; and means actuated by said follower arm in a predetermined position of said cam for disabling said retaining means and thereby causing the aperture in said plate to pass through theoptical axis at the same time that the aperture in said disk passes therethrough.

'1. In a shutter for controlling the passage of light through the optical system of a camera, thel combination of a shaft rotating at a substantially constant speed and lying parallel to the optical axis of the system, al disk on said shaft having a radius substantially greater than the distance between said shaft and the optical axis, said disk 1 having an aperture 'therein which passes through the optical` axis on each rotation of the disk, a screening plate situated in overlapping relationship with respect toisaid disk and having a normal position and a moved position, said plate being provided with an aperture which lies out of line with the optic-a1 axis when the plate is in either its normal or its moved position but which passes through the optical axis as the plate movesv from its normal to its moved position, spring said worm, a manipulative member normally holding said sector disengaged from said worm, and means actuable by said sector when released by said manipulative member for disengaging vsaid retaining means at a predetermined position of said sector in its travel through said worm to -thereby cause the aperture in said plate to pass through theoptical axis at the same time that the aperture in said'disk passes therethrough.

8. The invention as defined inclaim 6, wherein the selective engagement of said follower arm with said cam is controlled by a manipulative member normally holding said follower arm disengaged from said cam but enabling engagement of said follower arm with said cam upon mau nipulation thereof.

9. The invention as defined in claim 1, wherein means is provided for returning said screening member from its moved position to itsv normal position. f

10. The invention as defined in claim 1, Wherein means is provided for returning said screening member from its moved position to its normal position, said means being further effective to shield said aperture in said screening member during -the movement thereof from said moved position to said normal position.

1l. The invention as defined in claim 3, where in means lare provided for varying the position at which said driven element is effective for disabling said retaining means to thereby adjust the relative coincidence at which the apertures in said shutter and screening member pass through the optical axis.

12. In a shutter for controlling the passage of light through the optical system of a camera, the combination of a rotating shutter lying in the path of the light passingA through the system, said shtuter having an aperture therein which passes through the optical axis of the system on each rotation of the shutter, a'screening member likewise lying in the path of the light passing through the system and being provided with an aperture so positioned as to be registrable with said optical axis upon movement of said member, said screening member having a normal stationary position wherein said aperture lies out of line with said` axis, means for moving said member, and means operable in accordance with the movement of said shutter for so controlling said moving means as to cause the aperture in said member to pass through the optical axis at the same time that the aperture in said shutter passes therethrough.

13. In a'shutter for controlling the passage of light through the optical system of a camera, the combination of a shutter rotating at substantially constant speed and lying in the path of the light passing through the system, said shutter having an aperturetherein which passes through the optical axis of the system on each rotation of the shutter, a screening member likewise lying in the path of the light passing through the system and being provided with an aperture so positioned as to be registrable with said optical axis upon movement of said membensaid screening Y member having a normal stationary position as to cause the apeituite in said member to pass through the optical artis the same time that the aperture in said shutter passes therethrough.

it The invention as defined in claim 13, wherein means is provided for varying the speed of rotation of said. shutter.

l5. .T11 a shutter for controlling the passage of light through the optical system of a camera, the ation, ot an electi'ic circuit, a :cotating disk in the path light passing through the said dislt ,lo ving er1 apeisture therein 'passes through the optie-al etais of the sys-- tli eacln. te-tation ot the dists, an electric moto f in said elects/ic circuit i'oi' driving said disk, a screening member likewise lying o the path of the light passing tln'o'ugh this system and having a normal stationary position wherein light is preven d from passing through said system, means 'for moving said member, electromechanical means connected in said electric cii'cuit for .releasably CFR ietainine said member in its normal position, electric contact means operable in accordance with the movement of said disk for closing said electric circuit to said electromechanical means at the propel' moment to cause said moving means to move screening member so as to uncover the optical axis at the same time that the apeiftute in said shutter passes therethrough.

16. The invention as dened in claim 15, Wherein said electrical contact means are adjustable for varying the instant oi release of said screenine iioemlcei' in relation to the movement of said disk.

i7. The invention as dened in claim l5, wherein the electric motor is of the constant-speed type and Whei'ein there is further provided a Variableresistance means in the electrical circuit to vsaid motor for varying the speed of rotation of said disk.

SAMUEL ARONOFF. 

